This invention generally pertains to reactor vessel enclosures and more particularly to reactor vessel cavity structures including a refueling canal.
Refueling of pressurized water reactors is an established routine operation carried out with a high degree of reliability. For normal load requirements, refueling is provided on approximately yearly intervals. The complete refueling operation normally takes approximately four weeks.
In a number of nuclear containment arrangements the reactor vessel is positioned within a concrete cavity having an upper annular portion above the vessel which defines the refueling canal. The canal is maintained dry during reactor operations, however, during refueling of a nuclear power plant, the canal is filled with water. The water level is high enough to provide adequate shielding in order to maintain the radiation levels within acceptable limits when the fuel assemblies are removed completely from the vessel. Boric acid is added to the water to insure sub-critical conditions during refueling. The reactor vessel flange is sealed to the lower portion of the canal by a clamped gasket seal ring which prevents leakage of refueling water to the well in which the vessel is seated. This gasket is fastened and sealed after reactor cool down prior to flooding of the canal. Presently, the removable seal ring comprises four large diameter elastomer gaskets which are susceptible to leakage and must be replaced at each refueling operation.
A new concept has been developed by one reactor vendor for refueling pressurized water reactors in less than seven days. The advantages of rapid refueling are associated with a half yearly refueling cycle. This combination of rapid refueling and half yearly refueling, results in savings in fuel costs and increased availability of the nuclear steam supply system.
Rapid refueling reduces the number of operator actions in the refueling operation by eliminating certain operations and minimizing the number of components to be handled as explained in U.S. Pat. No. 3,752,737, issued Aug. 14, 1973 to E. Frisch et al and related patents cross-referenced therein. In fact, the number of individual handling operations are reduced by a factor of about six. Whereas current plants require approximately 2,500 handling operations, the rapid refueling system requires only 425. This has been achieved through design innovations which no longer require electrical lines to be disconnected in the operation, provide a quick opening reactor vessel closure, minimize fuel handling operations, expose the core in a single lift, and park the control rods in the head and upper internals.
While the above design innovations have simplified refueling operations considerably, the reactor vessel flange seal structure still provides a source of delay in establishing the base conditions for exposing the reactor core for refueling.
Accordingly, a new reactor cavity to seal arrangement is desired capable of expediting refueling operations.